just create a unit with the same pathing rules as your normal creatures, give them 999999 dmg, or however much damage you want them to have and tell them to run down the course.
When the referee unit gets stuck, they'll attack killing a blocking tower immediatley. Once that happens this trigger can execute, to keep the referee moving:
Trigger:
Referee Reset
Events
Unit - A unit Dies
Conditions
(Unit-type of (Killing unit)) Equal to Refereee
Actions
Set RefRestart = Referee
Unit - Order (RefRestart) to Move To <enter pathway here>
EDIT: If your TD is based on lives, set your Referee to Suicidal in their unit flags, then on your life counter set it to only trigger on a non-suicidal unit.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.